This invention relates to a nonlinear optical component having nonlinear optical properties such as wavelength conversion, bistability, amplification, EO modulation, phase conjugation, optical switching, and the like, particularly to a light frequency converter which converts a fundamental wave generated from a semiconductor laser into a second harmonic wave and emits, and a nonlinear optical material used therein.
Shorter wavelengths of semiconductor laser beams were attained by the development of materials having a broader band gap, for example, from group III-V compound semiconductors to group II-VI compound semiconductors. But the group II-VI compound semiconductor lasers have many problems to be solved until continuous oscillation at room temperature is obtained.
On the other hand, there is proposed a frequency converter applying second-harmonic generation (SHG). As a bulk crystal type nonlinear optical material for forming the frequency converter, there are known inorganic materials such as LiNbO.sub.3, potassium titanyl phosphate (KTP), etc. and organic materials such as 2-methyl-4-nitroaniline (MNA), 3,5-dimethyl-1-(4-nitrophenyl)pyrazole (DMNP). The organic nonlinear optical materials such as MNA and DMNP have advantages in that a nonlinear optical coefficient is large, a wavelength conversion efficiency is high and an optical damage threshold value is high (Japanese Patent Unexamined Publication Nos. 60-250334, and 2-29).
But the organic nonlinear optical materials have disadvantages in that the light absorption end is present at near 450 nm in both MNA crystal and DMNP crystal, and the second-harmonic generation in the blue light region is difficult.
In contrast, LiNbO.sub.3 and KTP have the light absorption end at 400 nm or less and can generate second-harmonic wave in the blue light region, but have a problem in that the frequency conversion efficiency is low in one order or more compared with the organic nonlinear optical materials. Other inorganic materials such as KH.sub.2 PO.sub.4 and NH.sub.4 H.sub.2 PO.sub.4 are disadvantageous in that their nonlinear optical susceptibility is considerably low for practical use and further these materials are deliquescent.
Generally speaking, organic nonlinear optical materials, which make use of polarization of the .pi. electron system, have a high nonlinear optical coefficient compared with the inorganic materials, and also are not deliquescent and excellent in damage threshold value, so that research and development thereof have widely been conducted in various fields. Recent results of such study are disclosed in detail in, for example, "Nonlinear Optical Properties of Organic Molecules and Crystals", vols. 1 and 2 (edited by D. S. Chemla and J. Zyss, published by Academic Press, Orlando, Fla., 1987).
However, there has not been found a material having a large nonlinear optical coefficient of such an extent as enabling satisfactory attainment with low output lasers such as semiconductor lasers. Thus, further development of novel materials is eagerly desired.